Liquid chromatography with electrochemical detection (LCEC) has become a widely accepted technique for trace analysis as evidenced by numerous technical publications in the area since 1970. Applications of oxidative LCEC have been primarily in the determination of trace organic species such as phenols, cathecolamines, pharmaceuticals, and antioxidants. Unlike oxidative LCEC, relatively few applications of reductive LCEC have been reported even though a large number of compounds are amenable to reductive detection.
Reductive LCEC suffers from several difficult problems which limit the method. Reduction of hydrogen ion can contribute to background current and limit the useful cathodic range. Simultaneous reduction of trace metals in the mobile phase can also contribute to high background current or alter detector performance. However, the most apparent limitation of reductive LCEC is the need to remove dissolved oxygen from the mobile phase prior to detection. Dissolved oxygen, at normal saturation levels (and in much lesser amounts), is both a direct and indirect interference for reductive detection. For example, in acid medium and at a mercury cathode (vs. SCE), oxygen is reduced in two steps: EQU O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O.sub.2 E.sub.1/2 =-0.5 V EQU H.sub.2 O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.2H.sub.2 O E.sub.1/2 =-1.0 V
and its presence results in intrinsically high background current and thus limited detection sensitivity. Also hydrogen peroxide, an intermediate in the oxygen reduction process, can detrimentally react with components in the mobile phase.
Various technical approaches have been developed to minimize the limitations of reductive LCEC posed by the presence of oxygen saturation of the mobile phase. These have included rigorous purging of eluent and sample solution with an inert gas; a method which is effective but cumbersome and kinetically slow. Other approaches which have been tried have involved, e.g., pre- or post-column addition of a chemical reductant (limited by reaction kinetics and solubility in chromatographic solvents); electrochemical reactors based on porous silver oxide to preferentially reduce oxygen prior to detection (not compatible with detection of easily reducible analytes); and signal conditioning techniques such as dual electrode detection and reverse pulse amperometry (which disadvantageously require special electronics to be effective). None of these techniques, however, has proved to be entirely satisfactory.